The invention relates to a multipurpose electrolysis cell which is bipolar-connected and is of high strucutral form for preferably high current loads of between 1 and 10 kA/m2 per individual bipolar cell. If the materials for the electrodes and the other cell assemblies are suitably adapted to the materials system in question, it can be used both in environmental technology for the electrochemical breakdown of inorganic and organic pollutants and in the chemical and pharmaceutical industry for producing inorganic and organic products. A particular application involves the production of peroxodisulphates and perchlorates.
Bipolar electrolysis cells of filter press design, comprising a clamping frame, the two electrode edge plates with supply conductors and any desired number of bipolar electrode plates, as well as peripheral equipment for supplying and discharging the electrolyte solutions and the cooling or temperature-control medium, are known in numerous forms and for a very wide range of applications. They may be of undivided form or may be divided into two-chamber or multichamber cells by means of ion exchange membranes or microporous diaphragms. The electrode or electrolyte spaces required can be designed as separate assemblies or may be integrated in the electrode edge plates or in the bipolar electrode plates.
Compared to the monopolar electrolysis cells which are of similar design, in filter press form, the considerable advantage of the bipolar electrolysis cells is that the current supply from the outside only has to be brought to the two edge plates, while the current transport in the individual bipolar cells takes place only from one side of the electrode plate to the other side, generally internally. For the most part, a simple bipolar electrode plate in which anode and cathode side consist of the same electrode material is not sufficient. In many cases, especially for multipurpose electrolysis cells, it is necessary to provide anodes and cathodes from different materials, preferably consisting of metal sheets. These can then be directly or indirectly connected to one another in an electrically conductive manner via contact bodies.
One possible embodiment of a bipolar multipurpose electrolysis cell of this type with a high height-to-width ratio which is required here, in order to achieve the “gas lift effect” for electrolyte circulation, as part of a gas lift electrolysis and reaction system which is of versatile design and can be used for a wide variety of purposes, is described in DE 44 38 124. This document describes an electrolysis cell structure which is optimized with a view to utilizing the lift provided by the evolved gases, with an overall height of 1.5 to 2.5 m. The bipolar electrode plates comprise electrode base bodies made from impregnated graphite or from plastics with feed and discharge lines machined in for the electrolyte solutions and the cooling medium, and electrodes and electrolyte spaces which are applied on both sides or, in the case of the graphite base bodies, are also integrated.
In this arrangement, the two electrodes, in the case of the graphite base bodies, are connected to one another in an electrically conductive manner via the latter, and in the case of the plastic base bodies are connected to one another in an electrically conductive manner by inserted contact elements. Such contact elements are arranged within the sealing surfaces which are covered by electrolyte frames made from elastic material. The contact is made as a result of the pressure during assembly.
Contact elements of this type arranged inside the plastic base bodies in the region of the sealing frames have drawbacks and risks particularly with high current intensities which are to be transmitted. For example, there is a risk of individual contact elements overheating, thus causing the entire bipolar unit to fail. The electrode base body, which is preferably made from thermoplastics, begins to soften at the overheated points, the pressure on the contacts drops and the inevitable result is an overload on the other contact elements. A further consequence may be melting of the baseplates, electrical spark-overs, uncontrolled discharge of electrolyte and also possible explosions as a result of the electrolysis gases then mixing. At any rate, the failure of a bipolar unit as a result of contact damage of this nature inevitably means the entire filter press cell is then out of action. The risk of such failure increases as the current load on the individual contact elements rises, the softening point of the plastic base bodies used decreases and the electrolyte temperature required rises.
A further drawback of internal contacts of this type is that in the event of leaks in the sealing system, electrolyte enters the press contact, where it leads to uncontrollable corrosion phenomena. This corrosion likewise causes the electrolysis cell to fail or be destroyed.
Therefore, bipolar electrolysis cells of this type with plastic base bodies have hitherto only gained acceptance for low to medium current loads of 100 to 1000 A and for low working temperatures.
It was also possible to eliminate these difficulties by dispensing with the use of plastic base bodies of this type. However, compared to the designs with plastic base bodies, the transition to one of the known all-metal designs for bipolar electrolysis cells, for example with both metal electrode sheets or cathodic and anodic half cells connected in an electrically conductive manner by screw connections to form the corresponding bipolar units also entails a number of drawbacks. For example, minimizing the current losses between the individual cells which are at different voltage levels and are connected to one another by the electrolyte lines requires special measures, since the electrical resistance in the connection lines for the electrolyte solutions is significantly lower than if electrically insulating plastic base bodies with the machined-in feeds and discharges for the electrolyte solutions are used.
In the numerous electrolysis cells which have been described hitherto, the electrodes used normally cannot be employed as metal electrode sheets which are simple to manufacture and are therefore also easy to exchange as part of a multipurpose cell. As soon as cooling channels or, when using perforated electrodes, electrolyte back spaces are required, welded designs are generally inevitable for the two half-cells, which often consist of different electrode materials or material combinations, of a bipolar unit. Particularly in the case of high-quality electrode materials and/or electrode materials which are difficult to process, the outlay on equipment involved in this is relatively high. Since the electrical contact between the two half-cells of the bipolar units is generally effected by a multiplicity of screw connections, assembly is significantly more complex than that of the cell designs in which this contact can be produced automatically by clamping together. Also, the transition to different electrode materials generally requires an altered design which is adapted to the materials properties.
An electrolysis cell for high current loads which is of monopolar design is described in DE 39 38 160.
The monopolar design has the fundamental drawback that a large number of individual cells have to be connected in series in order to approach a favourable voltage range for the current transformation (e.g. 200 V).
The electrolyte-side and current-side connection leads to high costs of the design.
A further drawback of the cells described is the design as a hollow body.
The abrasion of the active coating of the anode means that the entire anode body has to be manufactured again as new. The same applies to the cathode.
The pressing of the electrode hollow bodies causes deformation of the latter, and since they have no internal support (this would be extremely difficult to achieve in manufacturing technology terms), this leads to the electrodes being insufficiently plane-parallel. In extremis, this may lead to short circuit and therefore to the cell being destroyed and exploding.
These problems become more intense as the size of the cell increases and mean that only relatively small embodiments are produced, leading to high construction and operating costs with the drawbacks which have been outlined.
The desired versatile multipurpose electrolysis cell for high current loads can therefore scarcely be achieved on this basis.
The invention is therefore based on the problem of providing a bipolar multipurpose electrolysis cell which is constructed according to the filter press principle and has electrode base bodies which are made from plastic and in which good, operationally reliable contacting of the metal electrode sheets is ensured even at high current loads, while avoiding the drawbacks which have been outlined of the known technical solutions.
According to the invention, this problem is solved in the following way by the invention described in the patent claims: supply conductor plates and bipolar electrode plates with a height-to-width ratio of 30:1 to 1.5:1, preferably 10:1 to 1.5:1, are used, in which the metal electrode sheets and the electrolyte sealing frames project laterally beyond the electrode base bodies made from plastics and are connected both to vertical contact rails, which are arranged on both sides at a distance of 1 to 50 mm, preferably 5 to 50 mm from the electrode base bodies and, in the region of the electrolyte sealing frames, to the electrode base bodies, to form mechanically stable, bipolar electrode plates which can be fitted as independent units, the electrical contact between electrode plates and contact rails and the electrical insulation of two adjacent bipolar units with respect to one another being brought about by the electrolyte sealing frames, with simultaneous sealing of the electrolyte spaces when the electrode plates are clamped by means of the clamping frame as a result of the pressure. To maintain cell elements which can be handled individually, the cathode and anode sheets of a bipolar element are expediently screwed to the corresponding contact rails on one or both sides by means of countersunk head screws. This screw connection serves only to improve handling, however, and is to only a small extent responsible for the current flow, which is primarily optimized by the pressure of contact.
Since, therefore, the current contact is separated by an air gap from the electrolyte-carrying cell frame, leaks in the sealing system do not lead to the supply conductor failing in the medium term, since any electrolyte which escapes is drained, and as a result leaks of this type can be detected and remedied in good time.
In the case of the anode sheets, the metal electrode sheets consist of valve metals, preferably of titanium, which in the electrochemically active region are coated in a known way by active layers of precious metals, precious metal oxides, mixed oxides of precious metals and other metals, and other metal oxides, such as for example lead dioxide. Alternatively, other valve metals, such as tantalum, niobium or zirconium, may also be considered as supports for active layers of this type. However, lead-plated, nickel-plated, copper-plated steel or nickel-base alloys may also be suitable for particular applications.
In a particularly preferred embodiment, the anode sheets have a precious-metal application of solid platinum and are obtainable by hot isostatic pressing of platinum foil and titanium sheet.
The cathode material used is preferably stainless steel, nickel, titanium, steel or lead. Within the context of the present invention, cathodes made from high-alloy stainless steels of materials No. 1.4539 are preferably used, with an active electrode surface designed as expanded metal and resting on the back side directly on the perforated cathode frame part serving as a support.
The term perforated metal electrode sheets is to be understood as meaning in particular metal electrode sheets made from expanded metals. However, metal sheets which have been perforated in some other way or slatted electrodes may also be suitable.
The contact rails used are preferably contact rails made from copper, which may be tin-plated or silver-plated on the contact surfaces or may be coated with precious metals. The current contact surfaces of the electrodes are preferably provided with coatings of good conductivity, such as for example layers of platinum, gold, silver or copper, applied, for example by electrodeposition. The contact rails and the electrode contacts are preferably gold-plated or platinum-plated, and the current is transmitted as a result of the pressure contact formed as a result of clamping of the electrode assembly.
The design solution according to the invention, with contact rails which are arranged outside the plastic base bodies but still inside the clamping frame, however, can be utilized optimally for electrolysis cells of high current load and when using electrode materials which are expensive and/or of poor conductivity only if the high and narrow structural form according to the invention, preferably with a height of 1.5 to 3 m and a height/width ratio of 10:1 to 1.5:1 of the electrode plates is employed. Although similar cell dimensions have repeatedly been proposed for gas lift cells, in these cases it has only been with a view to optimizing the lift provided by the evolved gases in order to obtain a maximum gas lift effect.
In the present case, in combination with the contact according to the invention, the following advantages are produced even with electrodes without gas evolution: firstly, for an identical width of the contact rails, the contact area available proportional to the cell height rises, with the result that lower thermal loads are imposed on the contacts. However, the current transport from the contact surfaces through the metal electrode sheets is also promoted, since, for the same active electrode area, the same thickness of the electrode sheets and the same current load, the cross section, which is the determining factor for current transport, rises with the height of the electrode plates and, at the same time, the distance for current transport is reduced as the height increases. Under these boundary conditions, the electrical resistance falls and therefore the voltage drop in the electrode sheets falls by the square of the cell height. Therefore, with the same permissible voltage drop, with the narrow and high electrode plates which are to be used according to the invention it is possible to employ electrode sheets which are significantly thinner or less electrically conductive or to use significantly higher current loads. This is of great importance in particular in the case of perforated electrode sheets, in which a reduction in the cross section for current transport has to be accepted. Also, in the case of fitting of the cell assembly with thin sheet electrodes, any undulation in the sheet after pressing is compensated for, so that the electrode is made plane-parallel.
As a result of copper tubes which are soldered onto the outside of the contact rails, the contacts can be kept at or below room temperature by means of cooling water even in the event of high current loads. In this way, heating of the cell frame, of the sealing system and of the current contacts and the associated problems such as deformation and overheating are completely avoided.
The electrodes being plane-parallel with respect to one another represent a precondition for high current yields and uniform electrode corrosion.
The fact that the electrode plates are mounted so that they can move freely (float) in the sealing frame in the cell design described means that clamping and thermal expansion does not lead to deformation and curvature of the electrodes, so that excellent parallelism is achieved, and this can be stabilized still further as a result of a reduced pressure, described below, being applied to the anode back side, in a particular embodiment.
Finally, the height of the cell plays a role in the cooling of the highly loaded contact rails.
This is because it has been found that, particularly at high electrolysis temperatures in the gaps which are open at the top and bottom, an air flow is formed between the plastic base bodies and contact rails, which result in cooling of the contacts and the metal electrode sheets which project laterally beyond the plastic base bodies. This cooling effect likewise increases significantly with the cell height both as a result of the “chimney effect” and as a result of the growing “cooling area”.
It was thus possible to achieve the effect that the contacts, in particular at relatively high electrolyte temperatures, in a bipolar cell which is constructed according to the invention, adopt a significantly lower temperature than in the electrolysis cells with inner contact elements, in which, under comparable conditions, significantly higher temperatures are measured at the contact elements than in the interior of the cell. A further highly significant advantage of the distance between cell frame and contact web, which has already been mentioned, is that it is thus possible to drain off any small amounts of electrolyte which may escape. This is because if electrolyte penetrates into the contact gap, salt is formed and the contact deteriorates within a very short time.
A significant additional effect of the anode stabilization is achieved by the cooling medium.
The emerging cooling medium is taken off at a level below the height of the inlet. As a result, a reduced pressure, which can be adjusted by means of the level difference, is formed, and this pressure sucks the anode sheet onto the plastic base body and thus at the same time improves the plane-parallelism and prevents initial curvature of the anode in the event of pressure fluctuations in the cell. This measure makes it possible to achieve a very low electrode-to-electrode distance of 2 to 4 mm and therefore a low electrolyte. resistance and a high flow velocity.
The high flow velocity combined with a low mass throughput results in a high mass transfer to the anode surface, leading to a high yield of the anode product.